This invention relates to an electret, namely an elecrode plate having a polarized high molecular film, which is used in an electro-acoustic transducer which converts an electrical signal into an acoustic signal or vice versa through utilization of a change in electrostatic capacitance.
There has been put to practical use an electrostatic speaker in which an electrode plate electret having an electret deposited on one side of an electrode plate and a diaphragm are disposed in opposing relation to each other and an electrical signal is applied across them to vibrate the diaphragm in accordance with the electrical signal, thus converting the electrical signal to an acoustic signal. Also, there has been carried into practice an electrostatic microphone in which an electrode plate electret and a diaphragm are disposed opposite each other, the diaphragm is vibrated by an acoustic signal and the acoustic signal is converted to an electrical signal through utilization of a change in the electrostatic capacitance between the electrode plate electret and the diaphragm.
The following methods have heretofore been employed for the manufacture of the electrode plate electret used in such electrostatic type electro-acoustic transducers. A polarized high molecular film, commonly referred to as an electret film, is stuck to an electrode plate with an adhesive binder. With this method, however, the adhesive binder exerts bad influence on the electret film to degrade the characteristic of the electret and reduce the conversion gain of the transducer. Also, it is difficult to stick the electret film uniformly over the entire area of the electrode plate; hence, the electret film is liable to be mechanically distorted, which hinders its electrification and reduces its service life. Furthermore, this method is also disadvantageous in that the operation for sticking the electret film to the electrode plate is cumbersome and hence is poor in mass productivity and introduces dispersion in quality.
Another method that has been employed in the art is to weld the high molecular film to the electrode plate by fusion at a temperature close to the melting point of the film and then render the electret film on the electrode plate into an electret. With this method, however, the thickness of the high molecular film is made uneven by heating in the fusion welding of the film to the electrode plate, resulting in the electret film becoming unstable physically and hence short-lived. Moreover, this method involves heat treatment under high temperatures for heating the assembly up to the vicinity of the melting point of the high molecular film, and consequently this method is also poor in productivity.
It is an object of the present invention to provide an electrode plate electret which is easy to manufacture, suitable for mass production, stable physically and long-lived.
Another object of the present invention is to provide a method for the manufacture of an electrode plate electret with an electret film which does not require the use of an adhesive binder and achieves a uniform thickness of the electret film.
Another object of the present invention is to provide an electrode plate electret manufacturing method which enables mass production of electrode plate electrets that have an electret film of a uniform thickness, long service life and uniform characteristics.
Yet another object of the present invention is to provide an electrode plate electret manufacturing method which does not involve any treatment under high temperatures and hence is excellent in productivity.